Method and apparatus for coincidence imaging digital triggering

ABSTRACT

A method for operating a coincidence imaging system is provided. The method includes receiving samples from a detector output and determining an intersection of a first line corresponding to a baseline portion of the detector output samples and a second line corresponding to a pulse rise portion of the detector output samples.

BACKGROUND OF THE INVENTION

This invention relates generally to nuclear medicine imaging systemsand, more particularly, to methods and apparatus for providingcoincidence imaging in such systems.

Positrons are positively charged electrons that are emitted byradionuclides that have been prepared using a cyclotron or other device.These positions are employed as radioactive tracers called“radiopharmaceuticals” by incorporating them into substances, such asglucose or carbon dioxide. The radiopharmaceuticals are injected into apatient and become involved in such processes as blood flow, glucosemetabolism, and protein synthesis.

Positrons are emitted as the radionuclides decay. The positrons normallytravel a very short distance before they encounter an electron, and whenthis occurs, the positron and electron are annihilated and convertedinto a pair of photons, or gamma rays. This annihilation “event” istypically characterized by two features that are pertinent to positronemission tomography (PET) scanners, namely, each gamma ray has an energyof 511 keV and the pair of gamma rays are directed in substantiallyopposite directions. An image is created by determining the number ofsuch annihilation events at each location within the field of view.

A PET scanner may include two or more solid-state or scintillationdetectors to detect individual photons. Some known scanners include aplurality of detectors that define a ring around a volume of interest.Timing the detection of these events is used to identify the pairs ofphotons from a single annihilation. To facilitate effective operation,detection events should be able to be timed to ten nanoseconds or less.Each scintillation detector includes a scintillator that converts theenergy of each 511 keV photon into a flash of light that is sensed by aphotomultiplier tube (PMT). Coincidence detection circuits are coupledto the detectors and record only those photons that are detectedsimultaneously by two detectors located on opposite sides of some partof the patient. The number of such simultaneous events indicates thenumber of positron annihilations that occurred along a line joining thetwo opposing detectors. Within a few minutes hundreds of millions ofevents may be recorded to indicate the number of annihilations alonglines joining pairs of detectors in the ring. These numbers of eventsare employed to reconstruct an image using, for example, computedtomography techniques.

At least some known coincidence imaging systems use analog electroniccircuits to generate a timing signal resulting from the sharp rise inoutput voltage in the detection device when the detection device detectsan event. However, such circuits may have a limited accuracy when theevent rate in the detector is very high. Typical trigger circuits oftenfail to detect events occurring immediately after a preceding event, andwhen they do detect such events, they detect an event time later thanthe actual time.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method for operating a coincidence imaging system isprovided. The method includes receiving samples from a detector outputand determining an intersection of a first line corresponding to abaseline portion of the detector output samples and a second linecorresponding to a pulse rise portion of the detector output samples.

In another aspect, a trigger circuit for a coincidence imaging system isprovided. The circuit includes an analog-to-digital converter coupled toa detector output, a set of N accumulators coupled in a circuit parallelto the output of the analog-to-digital converter wherein N is apredetermined value, and a shift register coupled to the output of saidplurality of accumulators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary PET scanner system.

FIG. 2 is a graph of an exemplary event detection pulse output of thePMT shown in FIG. 1.

FIG. 3 is an expanded view of the exemplary event detection pulse outputshown in FIG. 2.

FIG. 4 is a schematic diagram of an exemplary trigger circuit that maybe used with the imaging system shown in FIG. 1.

FIG. 5 is a flowchart of an exemplary method that may be used toimplement the trigger circuit shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an exemplary PET scanner system 5 includinga gantry 10 which supports a detector ring assembly 11 about a centralopening or bore 12. A patient to be examined is positioned in front ofthe gantry 10 and is aligned with the central axis of the bore 12. Amotorized patient table (not shown) moves the patient into the bore 12in response to commands received from an operator workstation 15. Agantry controller 17 responds to commands received from operatorworkstation 15 through a communication link 18, such as, for example, aserial communication link to operate gantry 10.

In this exemplary embodiment, detector ring assembly 11 comprises aplurality of radiation detectors 20. An exploded view of one radiationdetector 20 illustrates that each radiation detector 20 includes a setof scintillator crystals 21 (for example, BGO crystals) arranged in amatrix and disposed in front of one or more photomultiplier tubes 22(PMT) or other light detector. Each PMT 22 produces an analog signal onat least one of a plurality of conductors 23 when a scintillation eventoccurs. A set of acquisition circuits 25 are mounted within gantry 10 toreceive these analog signals and produce digital signals indicating theevent coordinates (x,y) and the total energy the scintillation event.The digital signals from each acquisition circuit 25 are sent through acable 26 to a respective event locator circuit 27 that may be housed ina separate cabinet from gantry 10. Each acquisition circuit 25 alsoproduces a timing signal, which indicates when the scintillation eventtook place.

The event locator circuits 27 form a portion of a data acquisitionprocessor 30 that periodically processes the signals produced byacquisition circuits 25. Data acquisition processor 30 may have anacquisition CPU 29 that controls communications on communication link 18and a backplane bus 31. Event locator circuits 27 process and assemblethe information regarding each valid event into a set of digital numbers(e.g., event data packet) that include a time marker that indicates whenthe event took place and the position of scintillation crystal 21 thatdetected the radiation event. Each event data packet is transmitted to acoincidence detector 32 that also forms a portion of data acquisitionprocessor 30.

Coincidence detector 32 receives the event data packets from eventlocator circuits 27 and determines if any two of the event data packetsare in coincidence. Coincidence is determined by a number of factors. Inone embodiment, the time markers in each event data packet must bewithin 12.5 nanoseconds of each other (or some other value of timewindow), and second, the locations indicated by the two event datapackets must lie on a substantially straight line which passes throughthe field of view in scanner bore 12 and as described in more detailherein. Events that cannot be paired are discarded, but coincident eventpairs satisfying the coincidence requirements are located and recordedas a coincidence data packet that is conveyed through a link 33 (e.g.,serial link) to a sorter 34. In another embodiment, the timing signalsof two events which are deemed to be in coincidence are used tocalculate a time difference. The time difference is the time between thedetection of the first event of a coincidence pair and the detection ofthe second event of the pair.

FIG. 2 is a graph of an exemplary event detection pulse 200 output fromPMT 22 (shown in FIG. 1). Pulse 200 includes a baseline portion 202, apulse rise portion 204, and a decay portion 206. An x-axis 208represents time and a y-axis 209 represents an amplitude event detectionpulse 200. An analog-to-digital converter (not shown) of acquisitioncircuits 25 digitizes the voltage output from PMT 22 at a ratesufficient to obtain a plurality of samples 210, 212 and 214 duringpulse rise portion 204. In an exemplary embodiment, an event start 216is detected by measuring a voltage difference between successivesamples. Because noise may be a significant limitation in gamma raydetection, an event detection algorithm, executing on system 5, detectsthree successive voltage differences of greater than five percent of thefull-scale voltage. In an alternative embodiment, other algorithms mayselectably be used.

In this exemplary embodiment, a temporal accuracy of less than onenanosecond may be achieved by determining the time at which a straightline 218 fitted through pulse rise portion 204 intersects a straightline 220 fitted through baseline portion 202.

FIG. 3 is an expanded view of the exemplary event detection pulse 200(shown in FIG. 2). A difficult event detection situation occurs duringrelatively high event rates when a new event starts before the previousevent is completed. In an exemplary embodiment, an accurate estimate ofthe actual event time (e.g., the time the gamma ray entered detector 20)is obtained by determining the time where a straight line 302 fittedthrough baseline portion 202, prior to start of the event, intersects astraight line 304 fitted through pulse rise portion 204.

Pulse rise portion 204 may be fitted to the equation,y=m ₁ t+b ₁,

-   -   where m₁ is the slope and    -   b₁ is the y-intercept.

In the exemplary embodiment, a y-axis 306 (t=0) (shown as a verticalline) arbitrarily may be chosen on pulse rise portion 204, and slope m₁at that point may be determined as the average difference between themeasurements immediately preceding and following the chosen y-axis. Inan alternative embodiment, a time at which an event detector (notshown), using successive differences, detects the start of an event isselected as y-axis 306. Intercept, b₁, with this arbitrary y-axis is theaverage value of points immediately prior to and immediately following,y-axis 306. In this exemplary embodiment, the baseline slope may bemeasured similarly, for example using y=m₂t+b₂, where m₂ is the slope orthe baseline, and b₂ is the y-intercept. In an alternative embodiment,the baseline slope may be assumed to be zero. The baseline intercept,b₂, is then the average value of measurements made prior to the start ofthe rise portion 204.

The time of the start of the pulse is then calculated from:t=−(b ₁ −b ₂)/m ₁

Result, t, is the time between the start of the event (the point wherelines 302 and 304 intersect) and the arbitrary y-axis 306, in units ofthe sampling interval. This method may be embodied within a softwarecode segment executing on acquisition CPU 29 within system 5 or may beimplemented using dedicated firmware or hardware.

FIG. 4 is a schematic diagram of an exemplary trigger circuit 400 thatcan be used with imaging system 5 (shown in FIG. 1). Trigger circuit 400includes an analog-to-digital converter 402 including an input 404 andan output 406. Input 404 is coupled to an output of a photon detector,such as PMT 22. Output 406 is coupled to an input of a plurality ofaccumulators 408 that are coupled in parallel. An output of eachaccumulator 408 is coupled to an input of a shift register 410 thatincludes a plurality of register elements 412. Shift register 410includes a plurality of outputs, Q1, Q2, Q3, and Q4 that are selected togenerate proper delay times for computational accuracy of the pulsesamples taken during baseline portion 202 and rise portion 204 of pulse200. Q1 represents the sum of N values immediately prior to arbitraryy-axis 306. Q2 represents the sum of N values immediately afterarbitrary y-axis 306. Q3 represents the sum of N values centered onarbitrary y-axis 306. Q4 represents the sum of N values prior to thestart of rise portion 204.

Q1 and Q2 are transmitted to inputs of a computational logic circuit,such as an adder circuit 414 that combines Q1 and Q2 into an output, andin particular, the subtraction of Q1 from Q2 (Q2−Q1). Q3 and Q4 aretransmitted to a computational logic circuit, such as an adder circuit416 that combines Q3 and Q4 into an output Q3−Q4. An output of addercircuit 414 is coupled to an input of a look-up table component 418. Anoutput of look-up table component 418 and an output of the adder 416 arecombined by a multiplier 420.

In operation, analog-to-digital converter 402 transmits voltagemeasurements at a substantially constant rate to the set of Naccumulators 408 and shift register 410. Accumulators 408 each sum Nsuccessive voltage measurements and output the sums to shift register410, such that successive elements 412 of shift register 410 contain thefollowing summed voltage measurements:

${\sum\limits_{i = 1}^{N}\; V_{i}},{\sum\limits_{i = 1}^{N}\; V_{i + 1}},{\sum\limits_{i = 1}^{N}\; V_{i + 2}},{\sum\limits_{i = 1}^{N}\; V_{i + 3}},{\sum\limits_{i = 1}^{N}\; V_{i + 4}},{\sum\limits_{i = 1}^{N}\; V_{i + 5}},\ldots$

For example, in one embodiment where N=4,

-   -   sum1=v1+v2+v3+v4    -   sum2=v2+v3+v4+v5    -   sum3=v3+v4+v5+v6    -   sum4=v4+v5+v6+v7

When an event (i.e., coincidence event) is detected, for example, by athreshold method, four terms, Q1, Q2, Q3, and Q4 are extracted fromshift register 410 for processing. A delay in time from coincidencemeasurement to extraction at the Q4 element is determined by a responseof a threshold detector (not shown) and is selected to be sufficient toensure that in all circumstances, Q4 contains only values prior to riseportion 204. Q4, therefore is the sum of N samples from the ADC prior tothe start of the pulse rise, so represents the baseline immediatelyprior to the start of the rise. Q3 is the sum of N samples centered onsome arbitrary time, T, during the rise, so the average of these valuesshould equal the signal value at this arbitrary time. Q2 is the sum of Nsamples some time after the arbitrary time, T, and Q1 is the sum of Nsamples preceding the arbitrary time T by the same duration as Q1 lagsT. If N were chosen to be 1, then Q3−Q4 would be the signal rise fromthe baseline to time T, and Q2−Q1 would be the rise from the time Q1 wascollected to the time that Q2 was collected. Dividing Q2−Q1 by the timefrom Q1 to Q2 gives the slope of the rising part of the pulse.

The value of Q3−Q4 represents the value N*(b₁−b₂) and the value of Q2−Q1represents N values of (V_(i+k)−V_(j)) where k is the number of samplingintervals between the start of the Q1 and the Q2 sums. If N is selectedas a power of 2 (e.g., 2, 4, 6, 8, etc.) then a factor of N can beremoved by shifting or by selecting only the most significant bits forsubsequent processing. The resulting values B1=(b₁−b₂)=(Q3−Q4)/N andM1=(Q2−Q1)/N are the intercept and slope of the straight line fittingthe samples Q1 to Q3, where the y values are relative to the averagebaseline, b₂. The intersection of the rising portion of the pulse withthe baseline is the value of (Q3−Q4)/(Q2−Q1) is determined using bylook-up table 418 and multiplier 420. This is the time, relative to thearbitrarily defined, T, at which the rising slope intercepts thebaseline.

In an exemplary embodiment, using a sodium iodide scintillation detectoryields an event rise time of approximately eighty to ninety nanosecondsand a decay time of about eight hundred nanoseconds (with a two hundredthirty nanosecond half-time). Analog-to-digital converter 402, samplingat 200 MHz (i.e., every five nanoseconds) generates approximatelysixteen sample values during event pulse rise portion 204. The eventdetector (not shown) examines differences in every fourth sample, andtriggers if three successive such differences exceed a predeterminedlimit, such as five percent of full scale. Such a limit provides atrigger time about thirty to forty nanoseconds after the start of theevent, which becomes the arbitrary y-axis 306 (shown in FIG. 3).

The time calculation uses N=8. With an 8-bit analog-to-digital converter402, each of the Qn outputs has a range of ten bits, assuming, forexample, that the first half of the sampled rise is within the lowerhalf of full scale. The difference Q2−Q1 yields nine bits. The mostsignificant eight bits are used to access a look-up table component 418with a ten bit inverse value (i.e., one bit whole and nine fractionalbits), which in turn is multiplied, using multiplier 420, by the ten bitvalue, Q3−Q4. The resulting twenty-bit value contains eleven integralbits and nine fractional bits. However, this result is N times the truevalue of t, and so, if N=8, the result should be interpreted as fourteenintegral and six fractional bits. This time value may be entereddirectly as the event time.

A precise time marker may be generated at a time exactly T_(d) after thestart of the pulse. In the exemplary embodiment, the arbitrary y-axis306 (shown in FIG. 3) is about 30–40 nanoseconds after the start of theevent, accordingly T_(d) may be selected to equal, eighty nanoseconds(or sixteen sample intervals). Time value, t, calculated as describedabove, is the time from the start of the event to the arbitrary y-axis306, thus, the timing pulse should be generated at 16−t intervals afterthe arbitrary y-axis. (16−t) is calculated and the whole number partloaded into a countdown counter that counts down using the sample clockand generates a timing pulse at zero. This pulse (timed to the precedingclock before the desired time) is then provided to a programmable delayline, which has been loaded with the fractional part of (16−t), suchthat the pulse is delayed by the fractional part. The integral part ofthe result can be used to control a countdown timer that is partitionedinto three whole number bits, and three or more fractional bits.

FIG. 5 is a block diagram of an exemplary method 500 that may be used toimplement trigger circuit 400 (shown in FIG. 4). Method 500 includesreceiving 502 samples from a detector output and determining 504 anintersection of a first line corresponding to a baseline portion 202 ofthe detector output samples and a second line corresponding to a pulserise portion 204 of the detector output samples.

Exemplary embodiments of apparatus and methods that facilitate timingcoincidence events in a PET scanner are described above in detail. Atechnical effect of the timing apparatus and methods described hereininclude at least one of facilitating improving determination ofcoincidence pairs of positron annihilation events. The timing apparatusand methods include a plurality of logic devices and a processor thatallows for controlling a timing trigger and logic outputs.

It will be recognized that although the system in the disclosedembodiments comprises programmed hardware, for example, executed insoftware by a computer or processor-based control system, it may takeother forms, including hardwired hardware configurations, hardwaremanufactured in integrated circuit form, firmware, among others. Itshould be understood that the trigger circuit disclosed may be embodiedin a digital system with periodically sampled signals, or may beembodied in an analog system with continuous signals, or a combinationof digital and analog systems.

The above-described methods and apparatus provide a cost-effective andreliable means for facilitating generating a time signal to performcoincidence imaging. More specifically, the methods and apparatusfacilitate improving the accuracy of time signals during periods ofrelatively high count-rates. As a result, the methods and apparatusdescribed herein facilitate operating coincidence imaging systems in acost-effective and reliable manner.

Exemplary embodiments of coincidence imaging systems are described abovein detail. However, the systems are not limited to the specificembodiments described herein, but rather, components of each system maybe utilized independently and separately from other components describedherein. Each system component can also be used in combination with othersystem components.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for operating a coincidence imaging system, said methodcomprising: receiving samples from a detector output; and determining anintersection of a first line corresponding to a baseline portion of thedetector output samples and a second line corresponding to a pulse riseportion of the detector output samples.
 2. A method in accordance withclaim 1 wherein determining an intersection of a first line and a secondline comprises determining an intersection of a first line and a secondline wherein the intersection defines an event time.
 3. A method inaccordance with claim 1 wherein determining an intersection of a firstline corresponding to a baseline portion of the detector output samplesand a second line corresponding to a pulse rise portion of the detectoroutput samples comprises determining an intersection of a first linecorresponding to a baseline portion of the detector output sampleswherein the baseline portion is defined by a plurality of detectoroutput samples preceding the pulse rise portion, and a second linecorresponding to a pulse rise portion of the detector output samples. 4.A method in accordance with claim 1 further comprising determining thestart of an event based upon a voltage difference between successivesamples.
 5. A method in accordance with claim 1 wherein receivingsamples from a detector output comprises sampling a detector outputusing an analog-to-digital converter.
 6. A method in accordance withclaim 5 wherein sampling a detector output using an analog-to-digitalconverter comprises sampling a detector output using ananalog-to-digital converter at a predetermined rate that is sufficientto obtain a plurality of samples during the pulse rise portion.
 7. Amethod in accordance with claim 6 wherein N represents a predeterminednumber of accumulators, and wherein sampling a detector output using ananalog-to-digital converter comprises: summing N successive voltagemeasurements; and transmitting the sums to a shift register, such thatsuccessive elements of the shift register include:${\sum\limits_{i = 1}^{N}\; V_{i}},{\sum\limits_{i = 1}^{N}\; V_{i + 1}},{\sum\limits_{i = 1}^{N}\; V_{i + 2}},{\sum\limits_{i = 1}^{N}\; V_{i + 3}},{\sum\limits_{i = 1}^{N}\; V_{i + 4}},{\sum\limits_{i = 1}^{N}\; V_{i + 5}},\ldots$8. A method in accordance with claim 7 further comprising: detecting ascintillation event; and transmitting an output from the shift registerto a computational logic circuit.
 9. A method in accordance with claim 1wherein determining an intersection comprises: selecting a sampled pointon the pulse rise portion as the origin of a y-axis; and determining aslope of the second line using the equation, y=m₂t+b₂, where, m₂ is theslope of the second line; and b₂ is the intercept of the second linewith the y-axis.
 10. A method in accordance with claim 1 whereindetermining an intersection comprises determining a slope of the firstline using the equation, y=m₁t+b₁, where, m₁ is the slope of the firstline; and b₁ is the intercept of the first line with the y-axis.
 11. Amethod in accordance with claim 1 further comprising determining a timeof the intersection from the equation, t=−(b₁−b₂)/m₁, where, t is thetime of the start of the intersection and the y-axis in units of asampling interval, b₂ is an average of a predetermined number ofbaseline samples, b₁ is an average of a predetermined number of baselineportion samples and a predetermined number of pulse rise portionsamples, and m₁ is the slope of the straight line fitted through thepulse rise portion of the detector output samples.
 12. A method inaccordance with claim 1 further comprising generating a trigger pulse ata determined time interval after the start of the rise pulse.
 13. Amethod in accordance with claim 12 further comprising time-stamping eachtrigger pulse.
 14. A method for operating a coincidence imaging system,said method comprising: sampling a detector output that includes apulse, that has a baseline portion and a pulse rise portion, using ananalog-to-digital converter at a predetermined sample rate, such thatthe sample rate is sufficient to obtain a plurality of samples duringthe pulse rise portion; determining a slope of a first line fittedthrough a baseline portion of the detector output samples; determining aslope of a second line fitted through a pulse rise portion of thedetector output samples; determining an intersection of the first lineand the second line; and determining a time difference between theintersection and a pre-selected sample time of a pulse rise portionsample.
 15. A method for determining a coincidence event in acoincidence imaging system, said method comprising: defining a firstline corresponding to a baseline portion of a plurality of detectoroutput sample values; defining a second line corresponding to a pulserise portion of a plurality of detector output sample values; anddetermining an intersection of the first line and the second line, theintersection represents the time of the coincidence event.